The use of ferrite particles to produce heating in alternating magnetic fields is known in the art. As disclosed in U.S. Pat. Nos. 3,391,846 to White and 3,902,940 to Heller et al., ferrite particles have been used to produce heat where it is desired to cause chemical reactions, melt materials, evaporate solvents, produce gases and for other purposes.
PCT International Publication WO84/02098 (Application No. PCT/US83/01851 corresponding to U.S. Pat. No. 4,914,267) of Derbyshire discloses the use of ferromagnetic materials having the desired Curie temperature in electrically conductive layers to provide auto-regulated heating to the Curie temperature of the material upon application of an alternating current to the conductive layer of ferromagnetic material. The power applied to the ferromagnetic layer is in the form of an alternating current source which produces skin effect or eddy current heating in the continuous ferromagnetic layer. As the ferromagnetic layer reaches the Curie temperature, the permeability of the layer drops and the skin depth increases, thereby spreading the current through the wider area of the ferromagnetic layer until the Curie temperature is achieved throughout and the desired heating is achieved.
In my co-pending applications Ser. Nos. 07/242,208 and 07/404,621, incorporated herein by reference, I disclose the use of magnetic particles in combination with heat recoverable articles in an alternating magnetic field to effect heat recovery. While the systems disclosed therein are satisfactory for many applications, I have observed that in some situations the heating provided is not entirely as desired.
In some configurations and in some uses, it has been observed that, when the articles and magnetic particles are subjected to the alternating magnetic field, the initial heating to the Curie point of the particles is generally initiated satisfactorily. However, in some instances the desired continued heating is not maintained and in some instances the heating is uneven over the area of the article. In other applications, the cycling of the heating at or near the Curie point of the particles is not entirely as desired in that undesired cooling occurs before the particles again heat to the Curie temperature. Thus, desired sustained heating is not maintained.
In view of the above characteristics sometimes encountered in the prior art systems, it is an object of this invention to provide an improved system for heating with magnetic particles in alternating magnetic fields. It is an object of this invention to provide a system which produces desired continued heating in a wider range of articles and configurations of substrates. It is a further object of this invention to eliminate undesired temperature cycling sometimes encountered in the prior systems.